The proposed study will test the hypothesis that one of the major indirect pathways by which obstructive sleep apnea causes nocturia is the production of urinary natriuretic peptides and the subsequent increase in the production of urine. This study will simulate the physiological events created by obstructive sleep apnea by using negative pressure breathing in healthy volunteers. The primary aims of the study are: to determine the effects of negative pressure breathing on the excretion of sodium, water, atrial natriuretic peptide, and C-type natriuretic peptide; and to examine the relationship of age on these outcome variables. Forty healthy male and female adults (20 age 65+ and 20 under age 65) will be recruited to participate in a 330 minute experiment which includes 120 minutes of breathing against an air pressure of -12cm/H2O while in a recumbent position. Urine specimens for urine sodium, atrial natriuretic peptide and C-type natriuretic peptide will be collected at 30 minute intervals via urinary catheter. Procedures to control for gender, posture, hydration, sodium intake and caffeine intake are included in the design. Multiple regression analysis will be used to compare differences in the dependent variables across observations with and without negative pressure breathing, and to test for differences between older and younger subjects. Future studies will employ these methods and findings to test and validate a new method to differentially diagnose nocturia and/or obstructive sleep apnea by using a urine specimen.